You can use κατοικίδιος/-α/-ο as an adjective. For example: Είναι ένα κατοικίδιο ζώο (It's a pet animal) or Δεν υπάρχουν πολλοί κατοικίδιοι ελέφαντες στην Ελλάδα (There aren't a lot of pet elephants in Greece). But it's usually used as a noun and not as an adjective. More precisely, it's an adjective that has been transformed into a noun. It's called ουσιαστικοποιημένο επίθετο (noun-ified adjective) and there are plenty of them in Greek!